Keywords

3.1 The Task of Shale Gas Geological Survey

In recent years, China's consumption and production of conventional energy fuels have reached a record high level. Oil and natural gas still dominate the energy consumption structure. The sharp rise in domestic unconventional oil and gas production has basically balanced domestic oil and gas supply and demand (Jia et al. 2014). However, in the production of unconventional oil and gas resources, relatively cleaner shale gas resources have little contribution to the production, which is inconsistent with China's huge potential status of shale gas. Compared with the unconventional resources such as tight gas and coalbed methane that have achieved large-scale industrial production, the large-scale production of shale gas has made relatively slow progress, so it is necessary to seek further to realize the large-scale and effective development of shale gas resources.

Shale gas, as retained gas in source rocks (Li et al. 2012) and residual gas (Mou et al. 2016), is the accumulation of oil and gas in source rocks. In China, there are huge differences in the research basis and degree between petroliferous basins and sedimentary areas outside of petroliferous basins or where the degree of oil and gas exploration is very low. The former may have a certain working basis for basic data such as gas-bearing shale development series (including shale distribution and thickness), organic matter content and type, thermal evolution degree, gas display, rock characteristics and mineral components of gas-bearing shale.

However, for the latter, the above basic information is relatively lacking. In view of the research status in China, such as large differences in the basic work related to source rock research, weak foundation, no system, no detail and late start of research as shale gas, it is necessary to clarify its main tasks before the large-scale geological survey of shale gas.

Based on the author’s own actual experience in shale gas geological survey, combined with the analysis of domestic shale gas research status, existing problems and research trends, it is concluded that China’s shale gas geological survey has three basic and main tasks at present:

  1. (1)

    Clarify the basic geological characteristics of source rocks, including lithologic characteristics, sedimentary environment, sedimentary microfacies (rock) types and characteristics, organic matter types and contents, mineral composition, etc. Mainly through core and field observation and relevant sampling work, carry out the analysis of lithologic characteristics, depositional environment and TOC and Ro of shale of source rock and clarify the types and content of organic matter in source rocks. Further, analyze the types and contents of conventional minerals and clay minerals in source rocks, summarize the conventional mineral composition of source rocks and the variation law of clay mineral composition in the section and comprehensively study the basic geological characteristics of source rocks.

  2. (2)

    Clarify the temporal and spatial distribution law of source rocks, including their thickness, buried depth, fine distribution, area, etc. Mainly through the field, combined with seismic and drilling data, the distribution range and distribution law of source rocks are studied, and their thickness variation law, buried depth and variation trend are analyzed.

  3. (3)

    Optimize the prospective area and favorable area of shale gas reservoir. After completing tasks (1) and (2), we have the basic conditions to carry out the optimization of shale gas scenic spots and even favorable areas. On the basis of clarifying the scope of shale gas scenic spots, we can further carry out the research on the gas-bearing property of source rocks, reservoir capacity and resource potential analysis and select appropriate parameters in combination with geochemical indicators to further optimize the favorable areas or even dessert areas of shale gas.

3.2 Methods of Shale Gas Geological Survey

At present, China's shale gas geological research is in the stage of rapid development. Local exploration units represented by China Geological Survey are carrying out large-scale shale gas geological surveys, moving from theoretical learning to scientific practice, from key area survey to multi-regional and multi-type evaluation and from local test to large-scale investment. However, the geological conditions of shale gas in China are extremely complex, the theoretical research of shale gas geology is extensive, there are many problems, and the theoretical research is relatively backward. Combined with the basic purpose and task analysis of shale gas geological survey, the author believes that in addition to the guidance of basic geological theory, the key problem is to select appropriate technical methods.

In oil and gas exploration, although different disciplines have played different roles in identifying or making oil and gas breakthroughs, in the early stage of oil and gas exploration, lithofacies paleogeography research is indispensable and one of the key foundations for making breakthroughs in oil and gas resources. It is also a method of oil and gas exploration (Mou et al. 2010, 2011), and the same is true for shale gas (Mou et al. 2016).

At present, the mainstream view on the formation of oil and gas (including shale gas) is still the theory of organic genesis (Wang et al. 2003). Therefore, the formation of oil and gas is closely related to lithofacies paleogeographic environment from the generation and development of primitive organic life to the later death, burial, decomposition and migration and from the material of oil and gas reservoir to the formation of its preservation body. For shale gas, the study and reconstruction of lithofacies paleogeography can help clarify the scientific problems related to its occurrence carrier—organic-rich shale. Faced with shale, a restricted area of oil and gas exploration in the past, it is difficult to evaluate the target area of shale gas exploration, and the exploration risk and cost are high. Therefore, the understanding of lithofacies and paleogeography of shale gas target area has become more important, which should be paid attention to by geological investigators. However, oil and gas geologists and sedimentary geologists who conduct geological investigation and research on shale gas pay more attention to the geological characteristics of shale gas carriers (organic-rich shale), such as TOC, Ro, mineral composition. Secondly, it pays attention to the structural preservation conditions similar to conventional oil and gas reservoirs; however, the basic research on the fine sedimentary environment for the development of organic-rich shale, the carrier of shale gas and the research on the optimization of prospective area, favorable area and target area of shale gas as a key method is relatively few. A large number of studies show that the sedimentary environment not only controls the thickness, distribution area and organic carbon content of organic-rich shale, but also determines the sedimentary rocks type and mineral composition of the rocks, and the difference between rocks type and mineral composition determines the characteristics of physical property development of source rocks reservoir and then affects the accumulation of shale gas. Therefore, the sedimentary environment is the fundamental factor determining shale gas accumulation. Therefore, based on the study of regional sedimentary facies, the temporal and spatial distribution of favorable facies zones of source rocks can be clarified through lithofacies paleogeography map**, so as to provide the basis and direction for shale gas exploration (Mou et al. 2016). For the basic geological survey of shale gas, the research on the sedimentology and lithofacies paleogeography of shale gas source rocks and reservoir and taking this as a method to delineate the scenic spots, favorable areas and target areas should be the eternal theme throughout the whole geological survey of shale gas and the process of further exploration and development. In fact, the fundamental goal of a shale gas geological survey is to find prospective areas and favorable areas of shale gas, so as to provide a scientific basis for shale gas exploration and development (Mou et al. 2016). Therefore, the theory and method of “using superposition method to evaluate the prospective area, favorable area and target area of shale gas based on the map** research and map** of lithofacies paleogeography” should be the key technical method of large-scale shale gas geological survey. Research and practice have proved that the research and map** of lithofacies paleogeography can be used as the basic method and key technology and can guide the geological survey of shale gas.

3.2.1 Sedimentary Basin and Shale Gas

A large number of studies show that sedimentary basins play the most basic and important role in controlling the occurrence of a variety of sedimentary minerals and oil and gas (Wang and Li 2003), and the formation of sedimentary basins is controlled by tectonic activities and evolution. Zhang et al. (2003).

Gong et al. (2012) analyzed and compared the reservoir forming conditions of shale gas reservoirs in the USA, especially the control of structures and sedimentary basin types under their control of shale gas reservoir forming conditions and considered that the Foreland Basins are favorable places for shale gas formation and accumulation. The reason is that the lower strata of the Foreland Basin are usually stable craton shale deposits rich in organic matter, which provides sufficient material basis for the formation of shale gas. The upper strata of the Foreland Basin experienced multi-stage thrust folds, and the resulting tectonic thermal events provided thermal and dynamic conditions for the maturity of the lower source rocks and the generation of natural fractures, making the Foreland Basin an ideal place for shale gas accumulation. Li et al. (2013) also pointed out that the research about the control effect of structure on the distribution law of gas-bearing shale should be strengthened by analyzing the research status and problems faced by domestic shale gas. In other words, it is also the research on the type of prototype sedimentary basin. Shale gas reservoirs have the basic characteristics of integration of source and reservoir. Research shows that relatively high-quality reservoirs are often formed in a specific period with a small span, and the thickness may be only a few meters. For example, the most high-quality source rocks and shale gas development interval of the Fuling shale gas field of the Wufeng Formation-Longmaxi Formations at its bottom (Guo 2016), which is the first large shale gas field in China, and its material composition and lithofacies environment are significantly different from the similar lithology underlying and overlying it. What makes this obvious difference must be that there are some mechanism working. If we can accurately restore the types of sedimentary basins which controlled by structural evolution, clarify the distribution law of sediment filling process and lithofacies environment development under its control and master the internal law between them, it is possible to characterize the fine organic-rich shale section of shale gas development. This process is actually the analysis of sedimentary basins. Of course, it also has the analysis of fine sedimentary facies under its control, which will be discussed in detail below. In addition, in the process of analyzing the type of prototype sedimentary basin, we will have a very in-depth understanding and understanding of the geological background of the development of source rocks (such as the development degree of existing faults and microfractures, development characteristics of caprock, hydrogeological conditions, current pressure conditions), which will help to solve the research problems of shale gas preservation conditions. This is a great significance for the research on the important scientific problem of shale gas exploration and development, but it is not a high status in the research of shale gas preservation conditions in China.

The ultimate goal of the shale gas geological survey is to find shale gas reservoirs with industrial exploration and development value. Research and practice show that shale gas reservoirs with exploration and development value often exist in (or develop in) complete sedimentary basins. Most of those that have realized shale gas development or have great shale gas potential are Foreland Basins and the Craton Basins. Taking the USA as an example, nearly 20 shale gas blocks found in its 48 states widely develop gas-bearing black shale, most of which are developed in the Craton Basins and the Foreland Basins, such as the currently exploited shale gas reservoirs in the Foreland Basin on the west side of Appalachian-Quachita in the Eastern USA and the back-uplift basin group. At the same time, its potential shale gas blocks are also mainly distributed in the Foreland Basins and back-uplift basin on the east side of Rocky Mountain (USGS 2013). The tectonic evolution of the main development stages of these two sedimentary basins types and the paleogeographic conditions under their control, such as an anoxic reduction environment, suitable hydrodynamic conditions and less supply of terrigenous debris in most layers, control the formation of high-quality source rocks and shale gas with different genesis and maturity (Li et al. 2009). In China, taking the marine shale gas in Southern China as an example, the research shows a very large shale gas prospect in Sichuan Basin. The main development horizons are the Cambrian Niutitang Formation developed in the Craton Basin and the Late Ordovician Wufeng Formation-Early Silurian Longmaxi Formation in the back-uplift basin (Mou et al. 2010, 2011; Ge et al. 2014; Liang et al., 2014; Zhou et al. 2015) (Fig. 3.1). Or most researchers believe that the type of sedimentary basin is a Foreland Basin in the Late Ordovician-Early Silurian period formed on the eastern margin of the Yangtze Block due to the convergence of the Yangtze Block and the Cathaysian Block (Wang 1985; Liu et al. 1993; Xu et al. 1996; Yin et al. 2001, 2002; Su et al. 2007; Wang et al.

Fig. 3.1
figure 1

Distribution of Late Ordovician-Early Silurian tectonic framework and sedimentary basin types in the Middle-Upper Yangtze region (modified by Mou et al. 2010, 2016)

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In addition, the sedimentary pattern of the South China Basin is very different from the Mesozoic, and the sedimentary mechanism of the South China Basin is very different. Moreover, the sedimentary mechanism of the South China Basin is very different from the Mesozoic, and there are many tectonic changes. In the Mesozoic and even the Paleozoic, the basin experienced great tectonic deformation. The South China Basin has experienced tectonic uplift and Paleozoic, and the tectonic stability is very different. The sedimentary environment, tectonic setting and preservation conditions are extremely complex (Mou et al. 2010, 2011, 2016). The above factors have brought great difficulties to find relatively stable and well-preserved shale zones, which is the basic task of shale gas geological survey and the key to exploration and development in China. From the perspective of theory and practice, whether there is a complete shale gas basin is very important for shale gas development. Therefore, in the process of actual geological survey and exploration, development and production, it is very necessary to reconstruct a complete and fine prototype sedimentary basin of shale gas development in combination with the structural background. It is necessary to study the nature of the sedimentary basin, the size, shape, scale, etc. This work affects the work deployment of shale gas geological survey, and the calculation of shale gas resources and reserves, and further affects the exploration and development of shale gas. In the process of shale gas geological survey, even in the face of the basin that has been strongly transformed, on the one hand, we should study its evolution history and trace the prototype of the basin from the perspective of geological history evolution; on the other hand, from the perspective of “tectonic basin control”, we must study the plate tectonic background of the basin and the dynamic mechanism determined by it, so as to objectively and historically identify the basin type, so as to restore the complete sedimentary basin type in the development period of organic shale-rich shale conducive to the formation of shale gas.

3.2.2 Sedimentary Facies (Environment) and Shale Gas

The history of shale gas research and exploration practice in the USA for hundreds of years shows that one of the reasons why organic-rich shale in North America can make a major breakthrough in shale gas is that favorable sedimentary facies zones are generally developed. Although the mining area is small, it is in a favorable deep-water sedimentary environment. In addition, the favorable anoxic and stagnant sub-environment, which can be explained by the “Black Sea” detention quiet sea model and coastal upwelling model, has basically laid the foundation for the rich organic shale in its mining area and the high abundance of organic matter. Thicker thickness is a good material basis for shale gas development (Montgomery et al. 2005; Martineau 2007; Loucks and Ruppel 2007; Zhang et al. 2008a, b; Zou et al. 2010, 2013; Guo et al. 2012; Ma et al. 2012; Wang et al. 2014). For the geological survey, exploration and development of shale gas, source rocks is the basic material basis. Today’s research and exploration practice show that the main basis for the generation and occurrence of shale gas is the thickness of source rock, and the hydrocarbon generation potential of source rock formed in shallow sea shelf (including deep-water shelf area), slope, semi-deep sea and other environments is greater. If the thickness of the source rock is greater than 30 m, the gas remaining in shale is more conducive to enrichment and accumulation due to the influence of its own sealing layer; its shale gas development conditions are better (Li et al. 2011, 2012, 2016). Therefore, on the basis of reasonable theory, using appropriate working means to carry out the basic geological survey is the key to shale gas exploration in the Sichuan Basin with superior structural and sedimentary conditions. That is, we should start with the most fundamental geological situation research and start with the geological background of hydrocarbon source rocks development, that is, the basic paleogeographic research, and then follow the sequence of shale gas research, shale gas development conditions research and shale gas exploration and development without taking shortcuts.

Predecessors have carried out at least three rounds of systematic studies on sedimentary facies and lithofacies paleogeography with different scales and fine degrees in Sichuan Basin (Mou et al., 1992, 2010, 2011, 2016; Xu et al. 1993; Liu and Xu 1994; Ma and Chu 2008), preliminarily showing the Early Cambrian Qiognzhusi period (Niutitang Formation) during the deposition of high-quality source rocks in the Late Ordovician Wufeng period (the Wufeng Formation) and the Early Silurian long period (the Longmaxi Formation); these sets of source rocks are widely distributed and stably distributed regionally, which are potential shale gas development strata. The research shows that the basic geological research and determination of source rocks, especially the determination of high-quality source rocks, can play a very important role and significance in scientifically evaluating the oil and gas resource potential of the basin and deeply revealing the laws of oil and gas generation, migration, accumulation and enrichment (Lu et al. 2006; Hou et al. 2016).

In fact, the research and practice of China's energy resources show that the research and map** technology related to lithofacies paleogeography can actively and effectively serve the prediction and exploration of various minerals (Feng et al. 1997, 1999, 2005; Mou et al. 2010, 2011, 2106). Nowadays, the scope of prospecting work guided by lithofacies paleogeography research and related map** technology has been extended to almost all mineral resources, especially in the field of oil and gas. It has a more prominent effect in guiding oil and gas geological survey, prediction, exploration and development. It not only serves as basic geological science research for the geological survey of oil and gas resources. Moreover, it is often used as a key method and technical means to serve the prediction and exploration of oil and gas resources (Mou et al. 2016). It should be said that this is one of the main characteristics of lithofacies paleogeography in China, and it is also an important reason for the prosperity of lithofacies paleogeography.

Of course, the oil and gas resources mentioned above often refer to conventional oil and gas resources. As for the development of shale gas reservoirs, our views and suggestions are still the research of lithofacies paleogeography and its related map** methods and technologies, when we choose what theories and related methods and technologies to serve the geological survey or prediction, exploration and development of shale gas, can accurately and well complete the relevant tasks (Mou et al. 2016). Based on the study of regional sedimentary facies and the map** method of lithofacies paleogeography, the temporal and spatial distribution of favorable facies zones of source rocks can be clarified, so as to provide the basis and direction for shale gas exploration. Therefore, taking lithofacies paleogeography research and map** as the basic method and key technology can also provide guidance for the geological survey of shale gas (Mou et al. 2016).

In short, sedimentary facies control the basic geological conditions of shale gas development and its related properties and spatial distribution (such as the thickness, distribution area, organic matter content and type, mineral composition) for the geological survey of shale gas under stable structural units, and the detailed and systematic study of sedimentary facies and lithofacies paleogeography is an important basis and premise for the optimization of favorable areas for shale gas. It is also theoretical guidance and key methods and technologies (Mou et al. 2016). To carry out the geological survey of shale gas, we must first carry out comprehensive, overall and fine research and map** on the paleogeographic environment of source rocks, especially high-quality source rocks. Then, on this basis, focusing on the sedimentary microfacies belt or sedimentary environment conducive to the development of shale gas, comprehensively study the conditions or parameters of shale gas development or evaluation, select appropriate shale gas influence parameters and carry out the coupling superposition of relevant contour maps on the fine lithofacies paleogeographic map, so as to carry out the optimization of shale gas scenic spots, favorable areas and target areas. After such a systematic step, it is possible to grasp the key and key of shale gas geological investigation and research, so as to provide a more scientific basis for the final exploration and development of shale gas and realize the breakthrough of shale gas.

As mentioned above, ** of sedimentary facies, lithofacies and paleogeography have also been widely proved to play a basic and key guiding role in the success of oil and gas exploration industry. As Mou et al. (2011, 2012) said, when conducting a geological survey of oil and gas resources and further exploration and development in an area, the different disciplines will put forward corresponding suggestions and understanding from different angles. However, sedimentary geology, as one of the most basic disciplines, should be the basis and necessary content of research and work, but also a method. Its core is sedimentology and lithofacies paleogeography. The geological survey of shale gas is no exception. For the basic geological survey of shale gas in China, which is already in the stage of extensive development, the research and map** of sedimentary facies and lithofacies paleogeography are indispensable. It can be used as the key basic geological theory and method to provide guidance for the geological survey of shale gas, which is the first step of the basic work.

Theoretically, because the information of lithofacies paleogeography reconstruction comes from geological records, any information on the sedimentary environment in the sedimentary period of source rocks will be directly or indirectly branded and preserved in the geological stratigraphic records. However, due to the limited technical means and cognitive level of human beings, as well as the destruction and change of geological records, some congenital deficiencies or even sedimentary discontinuities are caused by later structural reasons. It brings great difficulties for researchers to obtain, interpret and retrieve these paleo-environmental information, resulting in great asymmetry in quantity, authenticity and reliability between the extremely rich and complex information in geological records and the parts available to researchers at present (Wang et al. 2003), which hinders the reconstruction research of lithofacies paleogeography and the geological investigation of shale gas resources to a certain extent. Therefore, on the basis of paying attention to the research and map** of sedimentology and lithofacies paleogeography, we should also pay attention to the research of other disciplines or theoretical technology, such as structural geology and the selection of geochemical analysis and testing methods, so as to more effectively carry out and complete the task and goal of shale gas geological survey. However, the understanding of shale gas geological survey with sedimentology and paleogeography as the core theory and key technical methods cannot be deviated. Extensive and comprehensive lithofacies paleogeography can be used as a guide for shale gas geological survey, and relevant lithofacies paleogeography map** technology can be used as a key technical method to realize the task of shale gas geological survey, so as to provide a solid foundation and scientific basis for further exploration and development of shale gas reservoir (Mou et al. 2016). The means of other disciplines (such as geochemistry) can only be an auxiliary means. The main purpose is to evaluate the geological conditions of shale gas development and then provide a reference basis for the optimization of shale gas favorable areas based on the study of lithofacies paleogeography and map**. This consensus and methodological understanding should be the product of the high combination of paleogeography and shale gas exploration and development practice (Mou et al. 2016). It should be emphasized that in the process of shale gas geological survey, sedimentology and lithofacies paleogeography are not only a regional, multi-information and multi-disciplinary comprehensive basic research, but also more importantly, they are a method of shale gas geological survey and a method of “looking for” shale gas.

3.2.4 Specific Methods and Steps

  1. 1.

    Restoration of prototype sedimentary basin

Organic-rich shale can be developed in rift basins (also divided into the intercontinental rift and intracontinental rift), passive continental margin basins, ocean basins, Craton Basins, trench-arc basin, system basins (such as intra-arc basins), residual basins, back-arc Foreland Basins, peripheral Foreland Basins and other qualitative basins. They represent the types of sedimentary basins under different plate tectonic backgrounds and dynamic mechanisms. It reflects the different evolution geological history from the Cratonic Basins extension stage, decline stage, residual stage and suture orogeny stage. However, most of today's sedimentary basins have been transformed by different tectonic activities in the later stage and show the type of superimposed sedimentary basin. Importantly, theory and practice have proved that a complete shale basin is very important for the development of shale gas. At the same time, those complete favorable structural units not only have high-quality source rocks required for shale gas development, but also have good caprock and other conditions which will be the key objects and areas for specific shale gas geological survey in the next step. It affects the specific deployment of shale gas geological survey.

Therefore, the first step of a shale gas geological survey is to restore the original properties of places rich in organic shale.

Firstly, with the idea of “tectonic controlled the development of basins “, it is not only necessary to study the plate tectonic background and the dynamic mechanism of the sedimentary basin during the development of organic shale in combination with the analysis of the relevant tectonic background. Secondly, we should study the evolutionary history of the sedimentary basin in this period, judge its evolution stage and trace the original nature of the sedimentary basin in this period from the perspective of geological history. Finally, on the basis of previous research results and practical basis, comprehensively and objectively identify and restore the types of sedimentary basins in the development period of organic-rich shale, judge their complete types of sedimentary basins and further screen or optimize the types of sedimentary basins that are more conducive to the development of high-quality source rocks for shale gas accumulation, such as the Craton Basin and the Foreland Basins. So as to restore their scope, the size, shape and scale lay the foundation for the following specific shale gas geological survey and provide a measurement reference and scientific basis.

  1. 2.

    Lithofacies paleogeography research and map**

  1. (1)

    Research on point

The research on point includes the sedimentary facies, sedimentary microfacies, rock types and characteristics, geochemical characteristics, mineral composition characteristics and vertical variation law of source rocks, in order to clarify the relevant basic geological characteristics of source rocks.

The specific approach is, through a large number of detailed field work, first to carry out the research on the fine lithology characteristics of source rocks from specific points, establish the backbone section or even “iron pillar” of the petrological characteristics of source rocks and then carry out the fine research on the types and characteristics of sedimentary microfacies (rock). If conditions permit, the observation and research of cores can also be strengthened. On this basis, through a certain degree of fine sampling, TOC, Ro and other parameters of source rocks reflecting shale gas development conditions are analyzed, various indoor identification, analysis, laboratory test and comprehensive research work are carried out, the variation law on the profile is summarized, and different gas-bearing source rock segments on a single point are divided. Through geochemical analysis, the types and contents of conventional minerals and clay minerals in different gas-bearing source rocks sections are analyzed, and their variation laws in profile and plane are summarized. Combined with the field and indoor comprehensive research results, the formation environment of different gas-bearing source rocks segments is further determined. Finally, establish a comprehensive histogram of the development characteristics of source rocks on the point, comprehensively clarify the various characteristics of the development of different gas-bearing source rocks segments from the point and complete the first goal of the shale gas geological survey.

  1. (2)

    Research from point to surface

The research from point to surface is mainly to conduct vertical sedimentary sequence and horizontal comparison, clarify the spatial distribution law of source rocks and the vertical and horizontal changes of shale gas development conditions and prepare single-factor maps of shale gas development conditions.

After comprehensively clarifying the development characteristics of different gas-bearing source rocks segments at each point, through the field profile connection and drilling well-connection profile, comprehensively analyze the thickness, TOC, Ro, mineral composition, occurrence and distribution of different gas-bearing source rocks segments from point to surface and then analyze their vertical and horizontal variation laws. If possible, it is also necessary to study the buried depth through seismic and detailed drilling data to further clarify the temporal and spatial distribution law of source rocks and their characteristic conditions, such as area, and prepare various single-factor maps of shale gas development conditions.

  1. (3)

    Lithofacies paleogeography research and map**

With the map** idea of “tectonic controlled the development of basins and sedimentary basins controlled the development of sedimentary facies”, on the basis of the above two studies, through the research and map** technology of lithofacies paleogeography, it shows the variation law of the thickness and range of source rock development, the evolution law of favorable sedimentary (micro)facies, the variation trend of buried depth, etc. and then comprehensively compiles the favorable sedimentary (micro)facies map and lithofacies paleogeography map of shale gas development.

Through the research of the above steps, we can clarify the temporal and spatial distribution law of source rocks, that is, the second task of the shale gas geological survey.